1. Field of the Invention
The present invention is directed to an over voltage protection circuit for at least one cell or battery.
2. Prior Art
Before filing this application, applicant conducted a patentability search to determine whether his invention was patentable over the cited references. These references are being provided to the U.S. Patent and Trademark Office. Of all the references, applicant believes that U.S. Pat. No. 5,695,886 to Dewan et al. is the most relevant.
Dewan et al. disclose xe2x80x9cLithium ion batteries are quickly gaining widespread popularity for powering portable electronic devices because of their superior energy density compared with more conventional battery systems such as nickel-cadmium and sealed lead-acid batteries. However, unlike the recent arrival of nickel-metal hydride batteries, lithium ion batteries require new charging regimes not used by any of the previous systems. Specifically, the lithium ion charge regime is a constant current to constant voltage regime. First, the lithium ion cells are charged with a constant current until a threshold voltage is reached, then the voltage is held constant by voltage regulation. The voltage regulation portion is critical. For optimum performance the voltage must be maintained within a very small tolerance, any less and the cells do not get fully charged, any more and the cells suffer significant overcharge.xe2x80x9d To address this issue, Dewan et al. developed an over voltage circuit design. The over voltage circuit design described by Dewan et al. is as follows:
The preferred over voltage control circuit [as illustrated in FIG. 1xe2x80x94prior art] comprises a comparator circuit 30, a voltage reference 32, a voltage divider 34, and a modifying switch 36 which modifies the voltage divider when the input signal is received. The comparator circuit 30 has a first input 38, such as a non-inverting input, and a second input 40, such as the inverting input. The voltage reference is connected to the first input, and provides a reference voltage Vref. The voltage divider is connected between the positive terminal 18 and the negative terminal 20, and comprises an upper resistance 42 and a lower resistance 44 which form a midpoint node 48 connected to the second input 40 of the comparator circuit 30. The modifying switch 36 comprises a modifying resistance 46, switch transistor 50, and pull down resistance 54. The switch transistor is connected in series with the modifying resistance 46, which is coupled between the switch transistor and the midpoint node 48. The switch transistor input 52 is connected to the input line 22, and while no signal is applied to the input terminal 24, the voltage at the switch transistor input is pulled low by the pull down resistance 54.
To illustrate the operation of the overvoltage disconnect circuit, assume that the battery is first connected to a charger of the first type. That is, a charger not designed to recharge a lithium ion battery, and thus no signal is applied to the input terminal 24. As no input signal is received, the switch transistor 50, which is preferably an N-channel Metallic Oxide Semiconductor Field Effect Transistor (MOSFET), is open since its input 52 is pulled low. That is, it acts as a high impedance. Thus, modifying resistance 46 is essentially floating and has no effect on the voltage divider 34.
The divided voltage produced at the midpoint node 48 is fed to the second input 40 of the comparator and compared to the reference voltage fed to the first input 38. For a given battery voltage Vbatt, the difference between the comparator inputs is a V1. When the voltage fed to the second input is lower than the voltage fed to the first input, the voltage at the output 62, which is fed to the overvoltage switch, also preferably an N-channel MOSFET, is at a high level such that the overvoltage switch is closed, or low impedance, thus allowing conduction. When the battery voltage reaches the first predetermined level, the resistances of the voltage divider are chosen such that the voltage fed to the second input will exceed the voltage fed to the first input. As a result, the voltage at the output 62 drops to a low level, thus switching off the overvoltage switch and disconnecting the battery. (Bracketed material added for clarity.)
In other words, Dewan et al.""s circuitry assists in the prevention of uncontrolled over voltage conditions. This circuitry samples cell voltage and disables cell charge if the voltage exceeds the rated voltage or desired voltage. At which time, the damage to the lithium cell may have occurred.
Dewan et al. admit that the value for the rated voltage or desired voltage is obtained through a microprocessor, computer program or memory system. This statement can be confirmed in the following paragraph found in the ""886 patent:
In practice it is typical that a lithium ion battery will have some auxiliary device 64, which will not be present in a conventional battery of the same form factor. As an example, it is common for a memory device to be included that contains information about the lithium ion battery, such as, for example, voltage regulation point, current level, charge capacity, etc. Since lithium ion is a relatively young battery cell chemistry, manufacturers of lithium ion cells use somewhat different chemical recipes, and often it is the case that cells manufactured by one maker require a first voltage regulation level, while those cells made by another maker require a second voltage regulation level. Since batteries may be constructed with either makers cells, it is imperative that the charger have correct charging parameters. As illustrated in [FIG. 1xe2x80x94prior art], it is possible to use the input terminal 24 for multiple uses. Here, the auxiliary device is a memory which is accessed by a charger of the second type when the battery is connected. The data is transferred relatively fast, and upon transference of data, the auxiliary line 26 of the charger is set to 5 volts, which will be sufficient to cause the switch transistor to close.
Along with the problems set forth above, there are other problems with the over voltage circuit protection devices illustrated and described by Dewan et al., and those shown and described in the other references. One of those problems is that those devices are too complicatedxe2x80x94the necessity of a modifying switch that must be used in association with a voltage divider. Another problem is that these devices require a microprocessor, computer program or memory system to properly operate. Applicant has solved these problems.
The present invention is directed to an overvoltage disconnect circuit for a lithium ion battery and/or cell. The lithium ion battery has at least one lithium ion battery cell having a rated voltage or a desired voltage, an input terminal, and being chargeable by a charger. The overvoltage disconnect circuit has (1) a switch unit, and (2 and 3) a first and second voltage dividers connected to (4) a comparator. The switch unit is in series with the lithium ion cell and the charger and the gate of the switch unit is connected to the comparator. The first voltage divider receives the voltage of the charger and generates a second charge. The second charge is proportionally below the voltage of the charger. The second voltage divider receives the voltage of the charger and generates a predetermined charge; the predetermined charge is proportionally below and sometimes less than the rated voltage or the desired voltage of the cell. The comparator compares the predetermined charge to the second charge. If the second charge is below the predetermined charge, the comparator transmits a an operational signal (which can be a non-signal) to the switch unit that allows the charger to continue charging the lithium ion cell; And if the second charge is equal to or greater than the predetermined charge, the comparator transmits the operational signal to the switch unit that disconnects the charger from charging the lithium ion cell.